1. Field of the Invention
The present invention is related to an atomic oscillator and a method for fabricating the atomic oscillator.
2. Description of the Related Art
An atomic clock (atomic oscillator) is regarded as a timekeeper with significant clock precision. Technologies for minimizing the atomic clock have been researched. The atomic clock is formed as an oscillator which refers to a transient energy amount of electrons forming an atom such as alkali metal or the like. Especially, a significantly precise value is acquired based on a transient energy of the electrons of the atom of the alkali metal in a state in which there is no disturbance. Compared with a crystal oscillator, it is possible to acquire frequency stability having significant higher figures.
There are some methods for the atomic clock. Among others, an atomic clock of a Coherent Population Trapping (CPT) method has frequency stability of approximately three figures higher than crystal oscillators in related arts, and can be expected to be ultra-minimized and to realize ultra-power consumption (see non-patent documents 1 and 2).
As illustrated in FIG. 1, the atomic clock of the CPT method includes a light source 910 such as a laser element or the like, an Alkaline metal cell 940 in which Alkaline metal is sealed, and a light detector 950 which receives a laser light passing through the Alkaline metal cell 940. The laser light is modulated, and is excited by simultaneously performing two transitions of the electrons in the Alkaline metal atom by sideband wavelengths which appear at both sides of a carrier wave being a specific wavelength. Transit energies in these transitions are constant. When the sideband wavelengths of the laser light correspond to a wavelength respective to the transit energy, a clearing response occurs, in which a light absorption rate in the Alkaline metal is reduced. The atomic oscillator has features in which a carrier wavelength is adjusted so that the light absorption rate due to the Alkaline metal is reduced, a signal detected by the light detector 950 is fed back to a modulator 960, and a modulation frequency of the laser light illuminated from the light source 910 such as a laser element or the like is adjusted by the modulator 960. The laser light is emitted from the light source 910, and irradiates the Alkaline metal cell 940 through a collimate lens 920 and a λ/4 plate 930.
Methods for fabricating the Alkaline metal cell in the micronized atomic clock with a Micro Electro Mechanical Systems (MEMS) technology are disclosed (see patent documents 1 through 4). In these disclosed methods, after an opening is formed on an Si substrate by a photolithography technology and an etching technology, a glass and the Si substrate are anodically bonded. Acnodically bonding is performed at 200° C. through 450° C. by applying voltage of approximately 250 V through 1000 V onto an interface between the glass and the Si substrate. After that, the Alkaline metal and buffer gas are input, and an opening part to form an upper surface is sealed by anodically bonding the glass. The Alkaline metal cell is formed by dicing material formed as described above for each cell.
Various methods are presented to enclose the Alkaline metal in a cell. A non-patent document 3 discloses a method in which Cs (Caesium) metal is directly dropped in vacuum to be sealed. Also, the non-patent document 3 discloses that a liquid solution mixing a BaN6 aqueous solution with CsCl is input into the cell, and a Cs metal is generated by being reacted at 200° C. after the cell is sealed. A non-patent document 4 discloses a method in which a Cs metal is generated by reacting BaN6+CsCl in an ampoule with a heater and is evaporated and transferred into a cell. A non-patent document 5 discloses a method in which after CsN3 is formed as a film in a cell by a general evaporation method, a UV light is irradiated and Cs and N2 are generated. A non-patent document 6 discloses a method in which after a Cs dispenser, which is stable in the atmosphere, is input in a cell, a laser light is irradiated onto the Cs dispenser alone to heat, and Cs is generated. Other methods may be presented.
In a case of sealing the cell by the anodically bond, oxygen, OH, H2O, and the like, which are generated by the anodically bond, react with the Alkaline metal in the cell. For example, in a case of Cs, since CsxOy and the like are generated, permeability of the laser light is fluctuated, and a frequency shift occurs. Thus, there is a problem in which short-term stability of the frequency is degraded.